Current Satellite-Based Augmentation Systems (SBAS) in the mid magnetic latitudes can provide a certain level of precision guidance for single frequency users; however, the ionospheric phenomena typically found at the equatorial latitudes significantly challenges current SBAS approaches to precision guidance. The three major phenomena causing the challenging accuracy distortions are the equatorial anomaly, depletion features (bubbles), and scintillations. Within approximately ±20 degrees of the magnetic equator is the so-called equatorial anomaly, which is the occurrence of a trough of concentrated ionization in the so-called F2 layer of the Earths' atmosphere. The Earth's magnetic field lines are horizontal at the magnetic equator. Solar heating and tidal oscillations in the lower ionosphere move plasma up and across the magnetic field lines. This sets up a sheet of electric current in the E region which, with the horizontal magnetic field, forces ionization up into the F layer, concentrating at ±20 degrees from the magnetic equator. This phenomenon is also known as the equatorial fountain. The equatorial ionosphere anomaly is an age-old problem that affects navigation systems.
In commercial global positioning system (GPS) based navigation systems, such as SBAS generally and Wide Area Augmentation System (WAAS), GPS Aided Geo Augmented Navigation (GAGAN), and Multi-functional Satellite Augmentation System (MSAS), for example, a phase of a GPS signal radio wave may be used to accurately estimate an aircraft position in single frequency (called L1) receivers of the GPS. To use the phase measurement, a user must know the phase ambiguity that theoretically is an integer multiplied by the wavelength. However, the ionosphere delay is substantial in the measurement and yet unknown which gives uncertainty to the ranging estimates to the satellites. The method used is the ground stations estimating the ionosphere delays from their measurements and uploading to the users (e.g., GPS receivers in an aircraft) in coarse sampled grids called ionosphere grid points (IGPs). A user receiver interpolates the IGP data for its line of sight (LOS) and thus estimates its ionosphere delay to each satellite.
In active ionosphere activity regions near the equator, however, the coarse grids often miss the peaks and valleys in the ionosphere charge distribution (called the residuals), and/or the user's LOS may have charge depletion zones (also called “bubbles”) that are not viewable from the ground stations. These distortion effects (also including scintillation) can cause considerable errors in the user's estimate of the ionosphere delay.
One technique for solving this problem in navigation system is to avoid measurements affected by depletion. In essence, no attempt is made to estimate the ionosphere delay through the depletion zone even when the measurements are available. Rather, the measurements are simply ignored. In particular, the user uses a depletion detection algorithm to identify and remove all the measurements that went through bubbles and averages the good measurements minus the ionosphere delay estimate from the IGPs to obtain an estimate for a phase ambiguity. Thus, this approach reduces the number of GPS satellites processed for navigation.
One problem with the approach, however, is that in equatorial regions, depletions may be prevalent, and if too many measurements are thrown out, there may not be enough remaining satellites with which to perform accurate ranging. This reduces the availability, the accuracy, and the continuity of the navigation system.
It would, therefore, be desirable to provide to a user accurate estimates of its ionosphere delays in the presence of depletions and appreciable residuals, which translate to a more accurate estimate of a user position.